1. Field of the Invention
The invention generally relates to immersion lithography for semiconductor fabrication and, more particularly, to a liquid-filled balloon for maintaining a liquid between a photolithographic lens and a wafer.
2. Background Description
A driving force in the continual improvement in complexity and cost of integrated circuits is uninterrupted enhancement of photolithography, particularly the ability to pattern smaller and smaller features. However, optical lithography is rapidly approaching physical barriers that limit the ability to continue scaling down.
Lithography uses radiation to transfer images onto a substrate coated with a material reactive to the radiation. Radiation in the form of light, for example, ultraviolet light, may be directed onto a mask (i.e., a photomask) defining a pattern. After shining through or reflecting from a mask the light is projected through a series of optical lenses and/or mirrors that shrink the image. The reduced image is then projected onto the workpiece. The workpiece may, for example, be a silicon or other semiconductor wafer covered with a radiation-sensitive photoresist. As the projected light hits the photoresist on the silicon wafer, it may alter the unmasked photoresist. Unaltered photoresist may then be chemically washed away, leaving patterned photoresist on portions of the wafer.
The minimum feature (w) that may be printed with a lithography system is determined by the well known Rayleigh equation:
                    W        =                                            k              1                        ⁢            λ                    NA                                    Equation        ⁢                                  ⁢        1.            where, k1 is the resolution factor, λ is the wavelength of the exposing radiation and NA is the numerical aperture.
NA may be determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens, as follows:NA=n sin α=d/(2f)  Equation 2.
where, n is the index of refraction of the medium surrounding the lens and α is the acceptance angle of the lens, d is the lens diameter and f is the focal length. As the sine of any angle is always ≦1 and n=1 (or approximately 1) for air, NA presents a clear physical limit for an air based system. However, a medium with a higher index of refraction than that of air may provide substantial resolution enhancement.
To achieve such enhancement, the medium between the lens and the wafer being exposed must have an index of refraction >1, have low optical absorption at the wavelength of light used for the lithographic process (e.g., a 193 nm wavelength for argon fluoride excimer lasers), be compatible with the photoresist and the lens material, and be uniform and non-contaminating.
For 193 nm exposure wavelength pure water meets all of these requirements, with an index of refraction n≈1.47 and absorption of <5% at working distances of up to 6 mm. Water can also be compatible with photoresist and photolithographic lenses and degassed and decontaminated for a high level of purity. Inserting n=1.47 into equation 2 and assuming sin α is 1, then the resolution limits for 193 nm immersion lithography are as follows:
                                                                                 W                =                                                                                                    k                        1                                            ⁢                      λ                                                              n                      ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      α                                                        =                                                                                                              k                          1                                                ×                        193                        ⁢                                                                                                  ⁢                        nm                                                                    1.47                        ×                        1.0                                                              =                                                                  k                        1                                            ×                      131.3                      ⁢                                                                                          ⁢                      nm                                                                                                                                              =                                                      32.8                    ⁢                                                                                  ⁢                    nm                    ⁢                                                                                  ⁢                    for                    ⁢                                                                                  ⁢                                          k                      1                                                        =                  0.25                                                                                        Equation        ⁢                                  ⁢        3.            
K1=0.25 is considered to be the theoretical minimum value for k1.
This 32.8 nm theoretical resolution represents approximately a 31% improvement over the line width attainable using air, i.e., 48.25 nm with 193 nm lithography.
A number of practical issues to implementing immersion lithography exist. The stage on a lithography exposure tool steps from location to location across the wafer scanning the reticle image for each field. To achieve high throughput, the stage must accelerate rapidly, move accurately to the next field location, settle, scan the image and then step to the next location all in a short period of time. Maintaining a consistent bubble-free liquid between the lens and the wafer is very difficult under these circumstances.
One approach has been to wholly or partially submerge the wafer stage, wafer and lens in a pool of water. The pool may be a recirculating pool or a stagnant pool. An issue with this approach is that submerging significant portions of multi-million dollar equipment requires significant re-engineering.
Another technique is to dispense water between the lens and the wafer with a nozzle. A suction port for liquid recovery may receive supplied liquid. Continuously maintaining a bubble-free even layer of water between the moving lens and wafer can be quite difficult using this technique, and larger topographical discontinuities, such as workpiece edges, complicate the engineering.
The invention is directed to overcoming one or more of the problems as set forth above.